The present invention relates to a method for purification of an exhaust gas, particularly an exhaust gas from diesel engines and an apparatus for purification of an exhaust gas.
Unburnt particulate substances in an exhaust gas from diesel engines, for example, solid carbon fine particles, liquid or solid high molecular weight hydrocarbon fine particles and the like (hereinafter referred to as "particulates") mostly have a particle size of smaller than 1 micron and are apt to float in the atmosphere and to be breathed into human bodies. Furthermore, since they contain carcinogenic substances, especially recently many problems are taken up in Japan and abroad and it is supposed that control of exhaustion be further tightened in the future.
Diesel exhaust gases contain NOx which is a harmful substance, in addition to the above-mentioned particulates and this substance is also regulated on its exhaustion. Production of NOx and the particulates are in the relation of trade-off, and there is the tendency that when a measure is taken to diminish emission of NOx, emission of particulates increases. Therefore, it is difficult to diminish both the NOx and the particulates only by the improvement of the operating system of engines, and removal of the particulates is carried out by physical trapping of them by a particulate trap provided in an exhaust gas line. However, when the trap is used for a long time, the trap is clogged with deposited particulates to result in pressure loss. Under the circumstance, as a promising method, it has been studied to periodically burn the trapped particulates and regenerate the trap.
The particulates contained in diesel exhaust gases comprise liquid or solid high molecular weight fine particles of a relatively low ignition temperature and solid carbon fine particles of a relatively high ignition temperature, and since the proportion of them varies depending on exhausting conditions (kinds of engines, operating modes, etc.), the ignition temperature of the particulates differs and, moreover, the ignition temperature of the solid carbon fine particles and liquid or solid high molecular hydrocarbon fine particles per se also differs depending on the exhausting conditions. However, in general, in order to burn and remove the particulates trapped in the trap to inhibit the pressure loss in the exhaust gas line, the trap and the trapped particulates must be heated to a high temperature (600-700.degree. C.) and this brings about the following problems.
(1) It is necessary for regeneration of the trap to keep the trap at a high temperature for ignition of the particulates, and this heat and the heat of combustion of the particulates after ignition cause increase in the temperature of the trap part, so that materials constituting the trap are molten and damaged.
(2) In regeneration of the trap, a heat source is supplied from one side of the trap but, since as mentioned above, the ignition temperature is high and the heat of combustion of the particulates is added thereto, a thermal stress in the trap increases due to the difference in the temperature between the ignited part and the unignited part, so that cracking occurs in the trap.